Field Effect Transistors (FETs) and High Voltage (HV) FETs require specially tailored well doping profiles and low well contact resistance. However, well contacts for thin-film silicon on insulator (SOI) are difficult to implement and typically have large parasitic capacitance, which limits device performance. The well contacts for thin-film SOI also typically have large resistance, which limits the robustness to high voltages.
More specifically, in standard SOI FETs, the source and drain are formed in a layer of silicon disposed on the silicon oxide-insulating layer. In SOI technology, if the body of an SOI transistor device floats, e.g., is not connected to a voltage source, the device characteristics and threshold voltage may vary with the switching history which the device experiences in actual operation. To cure such deficiencies, it is known to form a contact to the body of the device in order to allow the body to be connected to a voltage source. This may be done by use of a vertical gate line; however, known contact bodies have high resistance, which impart deleterious characteristics to the device.
By way of example, in known body contacts, the body contact is doped in the same concentration as that of the active region of a semiconductor device. This doping can affect many performance characteristics of the semiconductor device. For example, if the body doping concentration is increased in order to reduce the body-contact resistance, the threshold voltage of the device will increase in correspondence. Accordingly, under certain circumstances, a semiconductor device, with increased body doping to reduce body contact resistance, will tend to require higher gate voltage to conduct and to conduct less for a given voltage applied to the gate.
Yet another problem for body-contacted devices is the potential for the existence of a “sneak path” for current between the source and the drain adjacent to the device channel and beneath the region of the gate electrode which provides isolation between the body contact and the source/drain regions. When body doping is too low beneath this isolation region and adjacent to the source and drain regions, a parasitic channel can form between the source and drain which degrades operation of the device. This sneak path can be particularly exacerbated when the body-contacted device is operated at voltages, with respect to the substrate voltage, that tend to invert the body, providing a ‘back-gating’ action on this sneak path. Thus it is desirable to achieve low resistance the body contact, and to eliminate sneak paths, while maintaining low threshold voltage of the device.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.